84 



THE AMERICAN MONTHLY 



[May, 



The gelatin plate may be employed 

 in still another way, which, however, 

 preceded the one already mentioned 

 in the order of tinie and no doubt 

 paved the way for its invention. If 

 we dip a needle into a liquid culture 

 and draw it rapidly across the surface 

 of a layer of gelatin, we have dis- 

 tributed in this line or track a num- 

 ber of bacteria, the fewer scattered 

 along the way the better. In from 

 one to two days this track, at first 

 scarcely visible, becomes defined as 

 an opaque line, and under a low 

 power the colonies, descended from 

 single bacteria, may be seen distrib- 

 uted irregularly along this line. 

 These line cultures, as they might 

 be called, present all the characters 

 and variations which belong to the 

 isolated colonies, and in fact quite 

 frequently we are fortunate enough to 

 observe single colonies in the track 

 itself. By drawing 3 or 4 lines from 

 different cultures on the same plate 

 as has been done with these, we ai'e 

 enabled to compare their growth di- 

 rectly and also to determine whether 

 the culture from which each line was 

 inoculated was pure at the time. If 

 all the colonies occurring in a given 

 track are identical m appearance as 

 they enlarge, no matter what may 

 appear on the plate beyond the track, 

 we may safely assume that the cul- 

 ture from which the colonies origi- 

 nated was pure, in that it contained 

 but one distinct form or species. 

 This method was used on substrata 

 not so easily liquefied, such as agar- 

 agar and blood-serum. Dr. Rosen- 

 bach in a recent work* used these 

 line-cultures on agar-agar almost ex- 

 clusively, the method of isolating 

 germs not having been introduced. 

 In spite of the care with which the 

 w^ork w^as done and accuracy with 

 which the plates were drawn, we 

 cannot but feel that it must be done 

 over again with more recent methods 

 to be of permanent value. 



All of these methods must of ne- 



cessity be employed in the future in 

 the study of any micro-organism in 

 order that the work may be received 

 with confidence and foi*m the basis of 

 additional and more extensive inves- 

 tigations. 



* Mikro-organismen bei den Wundinfections krank- 

 heiten des Menschen. 



Abbe Condenser. 



We present this month an engrav- 

 ing of the very simple and inexpen- 

 sive mounting for the Abbe condenser 

 made by Mr. Zentmayer. The lower 



Fig. 13. — Zentmayer's Abbe Condenser. 



ring fits into the substage-ring, and 

 can be made to fit any microscope. 

 The milled head, seen below on the 

 right, moves the plate which carries 

 the diaphragms, one of which is 

 shown resting against the apparatus. 

 The arrangement for this purpose is 

 very neat and convenient. The cum- 

 bersome mounting of Mr. Zeiss is not 

 suited to the stands made in this coun- 

 try, and the lighter forms now made 

 by several of our opticians are quite 

 as efficient in every way. 



This condenser will fit the ' His- 

 tological ' stand, thus making the lat- 

 ter equal to any demands that may 

 be made upon it by the student of the 

 most evanescent microbes of disease. 



Lantern Transparencies. 



BY C. M. VORCE, F. R. M. S. 



Where a considerable number of 

 lantern slides are desired, as for dis- 

 tribution among co-workers, they can 



